The invention is concerned with devices for reading of position of a movable member, and more particularly devices wherein a digital read out of the position of a scale carried by the movable member is produced.
There are known numerous devices wherein displacement of a scale which is carried by the member and is constituted by references which are distributed at equal intervals, is read by a fixed detector, in which an optical system (this term designating not only radiation visible by the eye but also UV and IR) forms the image of a portion of the scale which occupies a predetermined position relatively to the detector onto the latter. Simple devices of this kind are generally incremental, that is to say do not directly determine the position of the scale but rather its displacement from a datum position. There also exist prior art devices which make an absolute determination (French patent specification No. 2241058). They generally use a detector which comprises a first system for approximation and a second system for carrying out the interpolation required to achieve a high degree of precision between a reference and the next successive (or next preceding) reference.
It is an object of the invention to provide an improved device for reading the position of a movable member (and thus, if required, to measure the displacement of this member in a predetermined direction). It is a more particular object of the invention to provide a device which is absolute (and thus does not at any moment require a re-checking with respect to an origin or datum position and is not subject to cumulative errors), and thus affords a large measuring range and a high degree of precision whilst remaining simple and permitting and enabling a high frequency of measurement.
According to an aspect of the invention, there is provided a device for reading of the position of a member movable with respect to another, comprising a scale carried by a first one of the members and having coded references distributed at equal intervals along the direction of relative movement of said members, each said reference being distinct from the other and identifying the serial number of the reference on said scale, and an optical system which forms, on a detector carried by the other member, an image of a portion of the scale which is situated at a predetermined location with respect to said other member and is of a length at least equal to said interval plus the maximum length of a reference in said direction, wherein the detector device comprises a linear matrix of photodetectors which are regularly distributed along said direction, and are operatively associated with electronic means constructed for identifying that coded reference whose complete image is formed on the matrix and for reading out the serial number of the photo element on which the image of a predetermined position in said coded reference is formed.
In this arrangement, there is no necessity for two different detector systems, and two different sets of references, one for identifying the reference of which the image is formed on the detector, and the other for interpolating between two successive references.
The matrix of photodetectors may in particular be constituted by a strip of photo diodes or photo transistors which is part of an integrated circuit constituting the electronic system. The code used is advantageously binary so that two levels are sufficient of illumination of the photo detectors.
The accuracy of measurement is conditioned by the accuracy of the engraving of the scale and the magnification of the optical system: this accuracy can be very high since, with a matrix of detector photo elements at a spacing of 25 .mu.m it is possible to achieve a precision of 1 .mu.m with an enlargement ratio of 25/1.